The present invention relates generally to hydraulic lash adjusters, and more particularly to a hydraulic lash adjuster (HLA) of the type in which there is both a high pressure chamber and a reservoir, or low pressure chamber.
Hydraulic lash adjusters (also sometimes referred to as "lifters") for internal combustion engines have been in use for many years to eliminate clearance, or lash, between engine valve train components under varying operating conditions, in order to maintain efficiency and to reduce noise and wear in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam through hydraulic fluid, trapped in a pressure chamber under a plunger. During each operation of the cam, as the length of the valve actuating components varies as a result of temperature changes and wear, small quantities of hydraulic fluid are permitted to enter the pressure chamber, or escape therefrom, thus effecting an adjustment in the position of the plunger, and consequently adjusting the effective total length of the valve train.
The cam operating cycle comprises two distinct events: (1) operation on the base circle and (2) valve actuation. The base circle event is characterized by a constant radius between the cam center of rotation and the cam follower, and during this event, no cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam center of rotation and the cam follower, which effectively transmits cam energy to open and close an engine valve. During the valve actuation event, a portion of the load resulting from the valve spring, the inertia of valve train components, and cylinder pressure are transmitted through the valve train and through the lash adjuster. The load increases the pressure of the hydraulic fluid within the lash adjuster pressure chamber, in proportion to the plunger area, and in typical hydraulic lash adjusters currently in commercial production, fluid escapes the pressure chamber between the plunger and the wall of the lash adjuster body. Such a device is referred to as a "conventional leakdown" lash adjuster. Although the present invention could be utilized in various types of hydraulic lash adjusters, it is especially adapted for use in an HLA of the conventional leakdown type, and will be described in connection therewith.
As noted previously, commercial HLA's of the conventional leakdown type have controlled the escape of fluid (or "leakdown") from the high pressure chamber solely by the fit of the plunger within the body, thus necessitating fairly close clearances therebetween. For example, it is common practice in the HLA art to centerless grind the outer surface of the plungers, and then select fit the plungers within the body bores to achieve a diametral clearance in the range of about 0.002 inches (0.0508 mm), which is considered a typical, nominal diametral clearance for a conventional leakdown type device.
There has recently been an increasing demand by vehicle manufacturers for an HLA having a relatively faster leakdown. Those skilled in the engine valve train art will understand that as the engine is warming up rapidly from very low temperatures, a conventional HLA may not be able to compensate quickly enough for the temperature-related growth in the length of the valve train components (especially, the exhaust valve). When such rapid growth occurs, and the HLA is unable to compensate quickly enough, the result is that the valve may remain open on the base circle which is generally recognized as being extremely undesirable.
One of the performance requirements for hydraulic lash adjusters is the ability to communicate lubricating fluid through the lash adjuster and out the ball plunger, to provide appropriate lubrication to an adjacent surface of a valve train component, such as the rocker arm. Specifically, it is important that the HLA be able to provide a fairly constant and consistent flow of lubrication fluid, without allowing air to be drawn through the meter passage in the ball plunger, and into the reservoir. An HLA having excellent lubrication metering capability is illustrated and described in U.S. Pat. No. 5,855,191, assigned to the assignee of the present invention and incorporated herein by reference. In the incorporated patent, there is provided a metering valve which defines a metering passage capable of communicating a small, controlled metering flow from the reservoir of the HLA to the external surface of the ball plunger. Unfortunately, there are now a number of engine applications in which the HLA is oriented generally horizontally, rather than vertically, thus limiting the ability to use the lubricant metering approach in the above-cited patent.